In the modern communications space, wireless access networks are increasingly popular, as they enable subscribers to access communications services without being tied to a fixed, wireline communications device. Conventional wireless access network infrastructure (e.g., base stations) is typically “built out”, by a network service provider, using a network-centric approach. Thus the build-out normally begins with major Metropolitan Service Areas (MSAs) using base stations located at the center of overlapping coverage areas or “cells”. The build-out, and corresponding wireless communications services, subsequently migrates outward from the MSAs to areas of lower population/service densities (e.g., urban to suburban to rural, etc.). At some point, usually dictated by economics, the build-out slows and/or becomes spotty leaving many individual wireless subscribers with unreliable or non-existent service.
On-frequency repeaters are known in the art for improving wireless services within defined regions of a wireless network (e.g., within a building or a built-up area). Such on-frequency repeaters are typically provides by the wireless network provider in order to improve signal quality in high noise or attenuation environments, where signal levels would otherwise be too low for satisfactory quality of service. In some cases, a wireless network provider may install a repeater in order to improve service in an area lying at an edge of the coverage area serviced by a base station, thereby effectively extending the reach of the base-station.
Prior art repeaters are part of a network-centric, view of the wireless network space, in that they are comparatively large systems provided by the network provider in order to improve wireless service to multiple subscribers within a defined area. As such, they form part of the network “build-out plan” of the network provider. These systems suffer the disadvantage in that an individual subscriber cannot benefit from the improved services afforded by the repeater unless they happen to be located within the coverage area of the repeater. However, there are many instances in which wireless subscribers may reside or work in areas where the coverage area of the wireless network is unreliable. Typical examples include mobile subscribers, and subscribers located in suburban and rural areas. Also, in-building coverage can be unreliable even within MSAs, depending on the size, location and construction of buildings and/or other obstacles. In such cases, it may be uneconomical for a network provider to build-out the network to provide adequate coverage, thereby leaving those subscribers with inadequate wireless services.
Accordingly, Applicant's co-pending U.S. patent application Ser. No. 09/809,218, filed on Mar. 16, 2001 and entitled Adaptive Personal Repeater, the contents of which are incorporated herein by reference, provides a method and apparatus that enables an individual subscriber to cost-effectively access high quality wireless communications services, independently of the location of the subscriber. The Adaptive Personal Repeater (APR) transparently mediates signaling between a subscriber's wireless communications device (WCD) and a transceiver (base station) of a wireless communications network. The APR includes a Directional Donor Unit (DDU) and a Subscriber Coverage Unit (SCU). The DDU maintains a network link with the base station of the wireless communications network. The SCU maintains a local link with the WCD within a personal wireless space of the APR. Total system gain is divided between., and integrated with, the DDU and the SCU, so that a separate gain and system control unit is not required. This division of system gain also enables high-performance on-frequency repeater functionality to be obtained without the use of high-cost components and building blocks.
As described in U.S. patent application Ser. No. 09/809,218, the APR represents a subscriber-centric solution for improving wireless services as required by one or more subscribers, and in a manner that is transparent to the network. However, in order to provide this functionality, it is necessary for the repeater to provide sufficient system gain in each of the uplink and downlink paths to compensate for propagation losses in these paths. On the other hand, if the system gain (in either the uplink or downlink paths) is too high, signal leakage between the two antennas will cause system oscillation. In principle, system stability can be obtained by ensuring that the antenna isolation is equal to or less than the system gain. However, in practice, the antenna isolation is difficult to predict (due to signal reflections in the environment within which the antennas are located), and will frequently change over time. Accordingly, conventional on-frequency repeaters are normally adjusted to provide a total system gain of about 10–15 db less than the antenna isolation, it order to provide an unconditionally stable system that precludes oscillation (even in a changing RF environment). This high (10–15 db) margin between antenna isolation and system gain is commonly achieved by limiting and sacrificing system gain, which significantly decreases the sensitivity (and thus efficiency) of the repeater.
Typically, antenna isolation and system gain are adjusted by service personnel during installation and set-up of the repeater unit, in order to achieve satisfactory performance. However, this is labor-intensive, iterative operation requiring skilled technicians using specialized equipment. This increases the cost and complexity of installing and maintaining the repeater, and thereby greatly discourages individual subscribers from acquiring a repeater for their personal use.
As is well known in the art, base stations and repeaters will frequently have overlapping coverage areas. Where one or more transmitters have overlapping coverage areas, signals from each of the transmitters will tend to interfere with one another. In particular, at locations within the overlapping coverage area where the phase difference between signals propagating from each transmitter approaches zero degrees, the signals will tend to interfere constructively, thereby increasing the signal power received by a subscriber's wireless communications device. Conversely, as the phase difference approaches 180°, the signals will interfere destructively to thereby reduce the signal power received by the subscriber's wireless communications device.
As may be appreciated, the extent of destructive interference will be a function of both signal amplitude and phase of each of the interfering signals. In particular, destructive interference increases as the phase difference approaches 180°, and as the difference between the signal powers approaches zero. Thus, when the signal powers are equal and the phase difference is 180°, the two signals will exactly cancel one another so that subscriber's wireless communications device will not receive any signal at all. Even when interfering signals do not completely cancel one another, destructive interference can sufficiently reduce the received signal power to prevent successful wireless communications. A region within which this occurs is referred to as a spatial null.
The geographical locations of spatial nulls within the coverage area of the network generally depend on the number, location and effective radiated power (ERP) of each transmitter serving an area, as well as the characteristics of the propagation paths from each of the transmitters. Because of the large number of variables, it is frequently difficult to predict the location of spatial nulls within the coverage area of a network. Accordingly, wireless network providers frequently do little to find and mitigate spatial nulls. Most subscribers are unaware of the existence of spatial nulls, and thus are confronted by unexplained interruptions in communications as they move from one location to another.
Accordingly, a method and system capable of enhancing wireless access by adaptively controlling system stability and mitigating spatial nulls, at a moderate cost, remains highly desirable.